2020托福听力练习：大迁徙导致遗传多样性（下载）

　　It's called the Great Migration—the journey of some six million African-Americans from the rural south to northern and western cities between 1910 and 1970. The cultural impact of the Great Migration has been well documented. But researchers have also started to look at its genetic implications. The most recent study finds that the migration had a significant effect on genomic diversity across the nation. That work is in the journal PLOS Genetics.

　　To estimate patterns of ancestry, researchers analyzed genetic data from nearly four thousand African-Americans who had participated in three medical studies.

　　Their findings confirmed historical records—genetic evidence showed that female slaves often gave birth to children fathered by white slaveowners. The genetic analysis found that 82.1 percent of the average African-American's ancestors came from Africa, while 16.7 percent were Europeans and 1.2 percent were Native American.

　　The team also could date when particular genes entered the mix. Native American genes were introduced into the typical African-American genome in the early 1600's. Genes from Europeans were for the most part introduced in the decades before and during the Civil War.

　　Researchers also found those still living in the southern United States have a greater percentage of African ancestry than those in the north or west. And European-Americans who now live in the south are more genetically similar to African-Americans in the north and west than they are to African-Americans currently living in the South—because Blacks with a greater percentage of European ancestry were more likely to move, especially early in the migration's history.

　　The analysis has implications for medical research and treatment. Most of the people in studies linking genes and disease have been white, thus potentially leaving gaps in medical information specific to other races. Filling in those gaps, along with better access to medical care, could help reduce the disparity in health experienced by many minority communities.